Biodiesel is fast becoming the substitute for petroleum-based fuel oil as the price of petroleum-based fuel oil continues to scale up high. One of the major problems associated with the use of biodiesel is its poor cold flow properties resulting from crystallization of saturated fatty compounds in cold condition.
In temperate countries, fuel oils are graded according to their cold flow properties: a summer grade fuel oil should exhibit pour point of not higher than 0° C., a spring or autumn grade fuel oil should exhibit pour point of not higher than −10° C. and a winter grade fuel oil should exhibit pour point of not higher than −20° C.
Palm-based biodiesel comprises palm products such as palm oil, palm olein, palm stearin, palm kernel oil, palm kernel olein and palm kernel stearin in crude or refined form and alkyl esters of palm products, particularly methyl esters of palm products. Palm-based biodiesel exhibits a relatively high pour point (approximately 15° C.) due to its high content of saturated fatty compounds; hence, its use is only suitable for tropical countries and unsuitable for temperate or cold climate countries where operational temperature is below its pour point.
Blending palm-based biodiesel with petroleum-based fuel oil such as diesel, kerosene and naphtha helps to lower down its pour point to a certain extent but the resultant palm-based biodiesel still does not qualify as winter grade fuel oil for use in extremely low temperature.
Pour point depressants or cold flow additives have been added to petroleum-based fuel oil to improve its cold flow properties. There is a wide range of pour point depressants available for use with petroleum-based fuel oil but normally they have limited effect on biodiesel.
U.S. Pat. No. 5,520,708 provides a method for reducing the crystallization temperature of a liquid fatty acid ester-petroleum distillate fuel blend comprising combining a petroleum distillate with an amount of a fatty acid ester of an animal or vegetable fat or oil selected from the group consisting of an isopropyl ester, a 2-butyl ester, a tert-butyl ester and mixtures thereof, wherein said amount is effective to lower the crystallization onset point of the blend by about 8°-20° C. below that of a blend comprising the same amount of methyl ester. In short, a fuel blend with lower crystallization onset point can be obtained by combining petroleum distillate with fatty acid ester of isopropanol, 2-butanol, t-butanol or mixtures thereof, instead of combining petroleum distillate with fatty acid ester of methanol. Judging by the fact that the preferred source of fatty acid is soybean oil, the preferred fatty acid ester is unsaturated fatty acid ester such as oleate and linoleate esters. The same result cannot be achieved if soybean oil is substituted by palm products as soybean oil and palm products have different fatty acid composition.
U.S. Pat. No. 6,203,585 discloses a biodiesel fuel composition having depressed pour point comprising:    (I) from about 95% to about 99%, by weight of the composition, of a methyl ester, or mixtures thereof, of fatty acids having from about 14 to about 24 carbon atoms; wherein said methyl ester has an iodine value from about 75 to about 125; and    (II) from about 1% to about 5%, by weight of the composition, of a copolymer additive comprising:            (A) from about 25% to about 75%, by weight of the copolymer additive, of a polymer comprising:                    (i) from about 70% to about 99.5%, by weight of the polymer, first repeating units, each derived from a C8-C15 alkyl methacrylate monomer; and            (ii) from about 0.5% to about 30% by weight of the polymer, second repeating units, each derived from a C16-C24 alkyl methacrylate monomer; and                        (B) from about 25% to about 75%, by weight of the copolymer additive, of a diluent which can be mineral oil, vegetable oil, polyol ester or mixtures thereof.        
The copolymer additive of this patent does not have the same pour point depressing effect on methyl ester of palm products.